Men who watch a lot of TV may have lower sperm counts, according to a new study. / Getty Images/Comstock Images

by Kim Painter, Special for USA TODAY

by Kim Painter, Special for USA TODAY

Healthy young men who watch a lot of TV and those who skimp on exercise have lower concentrations of sperm in their semen than guys who watch less and move more, a new study finds.

The study is small and does not prove cause and effect. But it adds to evidence that modern lifestyles may be contributing to possible declines in sperm counts in developed countries.

The findings also may offer some reassurance to active guys who have heard that exercise might decrease sperm counts, says Jorge Chavarro, an assistant professor of nutrition and epidemiology at the Harvard School of Public Health and a co-author of the study published Monday in the British Journal of Sports Medicine.

"Most of the previous research has focused on endurance athletes, such as marathon runners and professional cyclists," whose bodies may be under unusual stresses, Chavarro says. Biking, in particular, might overheat or injure the scrotum, research suggests.

The new study included 189 college men, ages 18 to 22, from Rochester, N.Y. Each gave researchers a semen sample and answered questions about how much time he spent watching TV and engaging in moderate to vigorous physical exercise. Among the results:

â?¢ Those who watched the most TV, more than 20 hours a week, had sperm counts 44% lower than those who watched none.

â?¢ Those who exercised the most, more than 15 hours a week, had sperm counts 73% higher than those who exercised the least, less than five hours a week.

â?¢ Other sperm characteristics, including swimming speed, were similar in all groups, and researchers could not say whether the differences in sperm count were big enough to have any effect on fertility.

Obesity and high-fat diets have been linked to lower sperm counts in other studies. But in the new study, the links between activity, TV and sperm counts held up even when weight and overall dietary patterns were factored in, Chavarro says. He and his co-authors suggest that metabolic changes related to inactivity and that heating of the scrotum caused by prolonged sitting in front of the TV might play roles - though studies on sperm counts in office workers who sit all day have yielded mixed results

Chavarro says TV still may be linked to important dietary factors, including exposure to lots of junk food advertising. Several other recent studies have linked prolonged TV watching to heart disease, diabetes and early death.

No one factor is likely to blame if sperm counts are falling, says Tina Kold Jensen, a professor of environmental medicine at the University of Southern Denmark, Odense. She co-authored a recent study linking reduced sperm counts to diets high in saturated fats. "I am sure that it is the combined lifestyle - no exercise, TV watching, cola drinking and fast food, that reduces semen quality," Jensen said in an e-mail.

Other researchers are looking for links with fertilizers, plastics, industrial chemicals and other environmental factors. Some scientists question whether sperm counts are falling at all: They blame changing lab tests, inconsistent study designs and other factors for the reported declines.